Vlll SCAPHOPODA. 



further anatomical details and embryology, the reader is referred to 

 the papers of Lacaze-Duthiers, Plate, Kowalevski and others, or to 

 the excellent resume by Simroth in Bronn's " Klassen und Ordnun- 

 gen." 



CHANGES OF THE SHELL WITH GROWTH AND AGE, ETC. 



The Scaphopods are like Ccecum, Rumina, Cylindrella and many 

 other Gastropods in successively truncating the shell posteriorly as 

 growth proceeds at the anterior end. The original apex is retained 

 only in extremely young individuals. This successive truncation is 

 necessitated not only by the excessive fragility of the early portion, 

 which would prevent its retention in any case, but by the necessity 

 for a larger anal orifice as the amount of water with its load of im- 

 purities increases with the size of the animal. 



The loss of shell substance is due occasionally to accidental break- 

 age, largely to chemical erosion by the water, but constantly to ab- 

 sorption by the mantle of the animal itself, such as occurs internally 

 in Neritidce and externally in roughly sculptured gastropods gener- 

 ally. These several causes, acting in varying combinations, produce 

 an extraordinary variety of forms, even among individuals of a single 

 species. The principal modifications are here tabulated : 



I. Apex simple, the orifice without slit, notch or tube (figures on 

 plate 22). 



II. Apex with an supplemental tube, built out. No notch or slit 

 ( P l. 18, figs. 4, 8). 



III. Apex with a V-shaped notch on the convex side, the orifice 

 usually surrounded by a short sheath formed of the inner 

 layer or lining of the shell left standing after erosion of the 

 prismatic layer outside of it (pi. 18, figs. 11,16, 17). 



IV. Apex with a long, narrow slit on the convex side (pi. 6, figs. 

 78, 79). 



V. A very long, straight linear slit on the convex side (pi. 19, 

 figs. 13, 21). 



VI. Slit on the concave face or on the side of the shell. Hetero- 

 schisma (p. 61), and occasional species or even specimens of 

 other groups, such as D. sericatum, inversum, alloschismum, 

 exdispar, pretiosum, etc., have the slit in an abnormal posi- 

 tion. The other characters of these forms show them to be- 

 long to various diverse groups. 



VII. Slit divided into a series of fissures (pi. 6, figs. 87, 89). In 

 D. (Schizodentalium) plurifissuratum, D. exuberans and D. 



